1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an optical pickup device suitable for a compatible optical pickup device for emitting several kinds of laser beams having different wavelengths to a recording medium.
2. Description of the Related Art
Currently, various optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have been commercialized and widely used. Further, recently, next-generation DVD standardization for recording and reproducing information using a blue-violet laser beam has been proceeding. In the next-generation DVD, information is recorded and reproduced using the blue-violet laser beam having a wavelength of about 405 nm. When the wavelength of the laser beam shortens, a higher density can be obtained.
Therefore, when the variety of optical discs increases, development of a so-called compatible optical pickup device capable of performing recording and reproduction on different kinds of optical discs is desired. In order to irradiate an optical disc with laser beams having different wavelengths, it is possible to employ an arrangement in which semiconductor lasers that emit laser beams having different wavelengths are separately disposed. However, when such arrangement is employed, spaces for separately disposing the semiconductor lasers and optical elements for guiding the laser beams to an objective lens are required. Consequently, the external dimensions become large and the number of parts increases.
Thus, an arrangement in which a plurality of laser elements having different emitting wavelengths are provided all together in a single CAN package has been studied. According to such arrangement, a space for disposing the semiconductor lasers can be reduced and an optical system can be commonly used among the laser beams.
However, when the plurality of laser elements are provided in the single CAN package as described above, a deviation in a direction perpendicular to the optical axis occurs between the optical axes of the laser beams according to arrangement gap between the respective laser elements. In this case, when the optical axis of the optical system is aligned with the optical axis of a laser beam, the optical axes of other laser beams deviate from the optical axis of the optical system. Consequently, in the case of recording and reproduction using the other laser beams, there arises a problem in that aberration of laser beams is produced on a recording medium or a photo detector to cause deterioration of optical characteristics.
Therefore, according to JP 06-131688 A, a birefringence element is disposed immediately after a semiconductor laser including several kinds of laser elements, and the optical axes of the laser beams are aligned with one another by the birefringence element to guide the laser beams to the optical system. That is, the plurality of laser elements are disposed in the same CAN package such that the polarization plane of a reference laser beam is orthogonal to the polarization plane of each of other laser beams. Then, disposed immediately after the semiconductor laser is a birefringence element that transmits the reference laser beam and refracts the other laser beams such that the optical axes thereof are aligned with the optical axis of the reference laser beam. According to this technique, with a refractive effect of the birefringence element the laser beams can be guided to the optical system located in the subsequent stage after the optical axes of the respective laser beams are aligned with one another. Thus, the laser beams having the wavelengths can be converged to a recording medium without aberration.
According to JP11-134702A, a diffraction grating is disposed immediately before a photo detector that receives reflected light beams from an optical disc, thereby guiding reflected light beams having different wavelengths to the photo detector. That is, three laser elements are disposed in the same CAN package. Laser beams having different wavelengths which are emitted from the respective laser elements are converged onto the disc by the common optical system. Then, reflected light beams from the disc are diffracted by the diffraction grating and converged onto the photo detector. According to the structure described above, the respective laser beams can be adequately converged to the photo detector. Accordingly, it is possible to obtain a detection signal with no fluctuation.
However, the conventional art described in JP 06-131688 A requires an additional birefringence element. Since the birefringence element is expensive, a problem occurs in that a cost of the optical pickup device as a whole increases. In addition, it is necessary to form in advance the laser elements such that the polarization plane of the reference laser beam is orthogonal to the polarization plane of each of the other laser beams. However, it is hard to form laser elements in which polarization planes of laser beams are made different from one another.
Refracting action of the birefringence element depends on the frequency. However, the wavelengths of the laser beams used for the compatible optical pickup device are in a close proximity to each other. Therefore, refraction angles when the laser beams having the different wavelengths are refracted by the birefringence element are not significantly different. For example, a wavelength difference between a laser beam for CD (780 nm in wavelength) and a laser beam for DVD (655 nm in wavelength) is only about 100 nm. As a result, refraction angles of those laser beams which are produced by the birefringence element become substantially equal to each other.
Therefore, when the optical axis of the laser beam for CD and the optical axis of the laser beam for DVD are to be aligned with that of a laser beam for next-generation DVD by refraction of the birefringence element, it is necessary to allow the laser beam for CD and the laser beam for DVD to enter the birefringence element with a state where the laser elements are approximated to each other to such degree that their optical axes are substantially aligned with each other. However, in manufacturing, it is nearly impossible to dispose the laser elements with the state in which they are approximated to each other to that degree. Thus, it is very hard to align the optical axis of the laser beam for CD (780 nm in wavelength) and the optical axis of the laser beam for DVD (655 nm in wavelength) with that of the laser beam for next-generation DVD by using the birefringence element.
According to the conventional art described in JP 11-134702 A, the reflected light beams having the different wavelengths are subjected to diffracting action by the only diffraction grating disposed immediately before the photo detector, thereby guiding the respective light beams onto the photo detector. In this case, when a variation from designed values occurs in arrangement or emission wavelengths of the laser elements for the different wavelengths, the reflected laser beams having the different wavelengths cannot be adequately guided onto the photo detector. To cope with that, a design of the diffraction grating needs to be suitably modified according to the variation from the designed values.
However, it is impractical to redesign, every time a variation from designed values occurs, the diffraction grating according to the variation. Therefore, in such case, practically there is no option but to use an existing diffraction grating without any modification assuming that no variation occurs. However, in this case, optical axis correction cannot be adequately performed.